Why would anyone sell below market value?

[Tony Otis]

New to real estate, would like to buy rental property in the next year or two.  I keep reading that I should be buying property at a discount.  Why would anyone be willing to sell me a house at less than what it's worth?  Are they just unaware of the value?

I see posts in the market place, like the one below, advertising 52% below market value.  Isn't market value defined as what someone is willing to pay?  Why would anyone want to sell something at a dollar amount less than what they could get from someone else?

http://www.biggerpockets.com/forums/517/topics/209799-zionsville---52-below-market-value---check-this-out

Perhaps just to get rid of the property quicker?

Thanks for reading, I'm sure there's something I'm missing here.